1. Field of the Invention
The present invention relates to a method of producing a semiconductor optical device.
2. Description of the Related Art
A semiconductor optical device with a buried heterostructure is known, the semiconductor optical device having a buried layer formed on both side faces of a semiconductor mesa part. Such a semiconductor optical device has a semiconductor mesa part including an n-type cladding layer, an active layer, a p-type cladding layer, and a p-type contact layer disposed in that order on one surface of a semiconductor substrate. For example, a semi-insulating layer of InP doped with iron (Fe) is disposed as a buried layer on both side faces of the semiconductor mesa part.
When a p-type cladding layer of the semiconductor mesa part is doped with zinc (Zn), as a p-type impurity, interdiffusion between Zn and Fe occurs at the interface between the p-type cladding layer and the buried layer. The interdiffusion between Zn and Fe may decrease the p-type carrier concentration in the vicinity of the interface and degrade the current blocking properties of the buried layer due to decrease of its resistivity. Then, this interdiffusion between Zn and Fe results in increasing the amount of leakage current flowing into the buried layer and decreasing the reliability of the semiconductor optical device.
Under these circumstances, for example, in a semiconductor optical device according to Japanese Unexamined Patent Application Publication No. 9-214045, a diffusion protection layer of an n-type InP is disposed between a mesa stripe including an active layer and a buried layer of an Fe-doped InP. Furthermore, a semiconductor optical device according to Japanese Unexamined Patent Application Publication No. 2003-60310 includes a diffusion protection layer of an InP doped with ruthenium (Ru). The n-type impurity or Ru added as a dopant to the diffusion protection layer prevents interdiffusion between Zn and Fe.